Diltiazem 120mg Extended-Release Conversion to Immediate-Release
For a patient taking 120mg once-daily extended-release diltiazem, convert to immediate-release diltiazem 30mg four times daily (total 120mg/day) to maintain equivalent total daily dosing. 1
Conversion Rationale
The American College of Cardiology guidelines specify that immediate-release diltiazem is dosed at 30-90mg four times daily, while extended-release formulations are dosed at 120-360mg once daily. 1 The key principle is maintaining the same total daily dose when converting between formulations. 2
Practical Dosing Schedule
- Immediate-release: 30mg four times daily (e.g., 8 AM, 12 PM, 4 PM, 8 PM) provides equivalent 24-hour coverage to 120mg extended-release once daily. 1
- Alternatively, 40mg three times daily (total 120mg/day) may be used if four-times-daily dosing is impractical, though this is less ideal for maintaining consistent blood levels. 1
Important Pharmacokinetic Differences
- Immediate-release diltiazem has a half-life of 4.5-12 hours, requiring multiple daily doses to maintain therapeutic levels, whereas extended-release formulations provide consistent 24-hour drug delivery. 3
- The extended-release formulation maintains steady plasma concentrations above the therapeutic threshold (≥40 ng/mL) throughout the dosing interval, while immediate-release requires frequent dosing to avoid trough periods. 4
Clinical Considerations for Conversion
When This Conversion Is Appropriate
- Patients unable to swallow extended-release capsules (which cannot be split or crushed) may require immediate-release tablets. 5
- Hospitalized patients requiring more flexible dosing or those transitioning to intravenous therapy may benefit from immediate-release formulations. 1
Monitoring After Conversion
- Reassess blood pressure and heart rate within 1 week after converting to immediate-release to ensure adequate control throughout the 24-hour period. 3
- Watch for breakthrough symptoms (elevated blood pressure, angina, or rapid heart rate) particularly in the hours before the next immediate-release dose. 3
Critical Safety Warnings
Absolute Contraindications Apply to Both Formulations
- Do not use diltiazem in any formulation in patients with second- or third-degree AV block without a functioning pacemaker, sick sinus syndrome without a pacemaker, decompensated systolic heart failure, severe left ventricular dysfunction, cardiogenic shock, or Wolff-Parkinson-White syndrome with atrial fibrillation/flutter. 1, 3
Drug Interaction Alerts
- Diltiazem is both a CYP3A4 substrate and moderate CYP3A4 inhibitor—exercise caution with apixaban, rivaroxaban, simvastatin (maximum 10mg daily), cyclosporine, and other CYP3A4 substrates. 3
- Avoid concurrent beta-blocker therapy due to increased risk of profound AV block, symptomatic bradycardia, and heart failure. 1, 3
Common Pitfall to Avoid
Do not assume 120mg once-daily extended-release can be replaced with 120mg once-daily immediate-release—immediate-release diltiazem dosed once daily will not provide adequate 24-hour coverage due to its shorter half-life, resulting in loss of therapeutic effect during trough periods. 1, 3 The immediate-release formulation requires division into multiple daily doses (typically four times daily) to match the pharmacokinetic profile of extended-release. 1